According to the prior art, a stiffened composite panel is made of a thermosetting resin with continuous fiber reinforcement. It includes a skin that is stiffened, firstly, by longitudinal stiffeners called stringers and secondly by stiffeners extending along a direction perpendicular to said longitudinal direction, generally on the same side. The techniques of implementation of composite materials do not make it possible to apply such cross stiffening with single lengths of continuous stiffeners. Thus, to achieve that characteristic, when the stiffeners are placed on the same side of the panel, a first series of stiffeners, for example longitudinal stiffeners, are simultaneously co-cured, glued or simultaneously draped with the skin; for their part, the stiffeners that are orthogonal to them are assembled onto the skin by rivet type fasteners with cleats that are themselves riveted or glued to the skin. That panel manufacturing mode implies that it is made in several distinct assembly steps. Besides, it implies specific problems relating to the low peening resistance of the composite material which makes it necessary to oversize the rivets and increase the thicknesses at the fasteners, and thus increase the mass of the finished panel.
To remedy that drawback of the prior art, when the panel does not comprise an aerodynamic side, the continuous orthogonal stiffeners are advantageously placed on both sides of the skin. While that configuration makes it possible to achieve cross stiffening by continuous stiffeners without using intermediate cleats or fasteners, as said stiffeners are placed on each side of the thickness of the skin, making the stiffeners by direct drape forming or by adding them by simultaneous curing is very delicate since it makes it necessary to handle a non-consolidated pre-form while turning it over and to place and to position it in a complex tooling. Such a panel has a high manufacturing cost and the many manufacturing steps constitute as many sources of dispersion liable to lead to manufacturing quality defects.
The document EP 1 149 687 describes a composite panel stiffened by orthogonal stiffeners fixed by welding on the same side of the panel.